Stereo Bass from AV Processor or Receiver?

Question:I want to use two subwoofers and keep the bass stereo for recordings that, in fact, have phase differences in the low frequencies. So I am looking for a processor or receiver that, when all the main speakers are set to “small,” sends the left-side bass to the left subwoofer, the right-side bass to the right subwoofer, and splits the center-channel bass. Also, if any product actually does this, what happens when you run the auto EQ/room correction? Would the auto EQ make the subwoofers mono?

– Richard SeveranceSt. Paul, MN

Answer:I know of only two AV preamp/processors that can do what you want—the Denon AVP-A1HDCI and Classe SSP-800 reviewed here, both of which have three subwoofer outputs that can be configured as you suggest. However, the Denon lists for $7500, and the Classe lists for $9500, so they’re not for those on a budget.

Other than these two, I was unable to find any AV receivers or preamp/processors that do what you’re asking. I looked for models with two subwoofer outputs, such as those with Audyssey Sub EQ HT, which is part of the Audyssey suite of room-correction algorithms that deals with two subs. A list of current products that include Sub EQ HT can be found here. These include Denon, Integra, and Onkyo AVRs and pre/pros along with the Marantz AV8801 pre/pro.

I asked these companies if any of their products do what you want. I didn’t hear back from Onkyo and Integra, but I confirmed that all the Denon and Marantz models treat the two sub outputs as one—that is, they sum all the bass frequencies to mono and send that signal to both outputs.

SECRETS head honcho John Johnson has come up with an interesting way to keep the left and right bass separate in his home-theater room. He has the Classe SSP-800 for movies, but he prefers to use the Classe CP-800 stereo preamp, which also has independent left and right subwoofer crossovers and outputs, for 2-channel listening. He connected the HDMI output from an Oppo BDP-105 universal-disc player to the SSP-800 and the XLR balanced-analog audio outs to one stereo input on the CP-800. Then, he connected the front left and right XLR pre-outs from the SSP-800 to another input on the CP-800.

When he listens to 2-channel SACDs from the Oppo, the SSP-800 isn’t in the signal path at all, and he hears stereo bass from two subs crossed over from two electrostatic speakers at 60Hz. For Blu-ray movies, the HDMI signal goes through the SSP-800, which sends the front left and right channels directly to the CP-800, which reproduces stereo bass in those channels. The SSP-800 is set to mono bass, and the low frequencies from the center and surround channels are redirected to a third sub. (Note: if you don’t want to use a separate stereo preamp for 2-channel audio, you can set the SSP-800 to do exactly as you want—stereo bass from the front left and right channels, and a third sub channel for the bass from the center and surrounds.)

John wrote about a similar setup here. In that case, he used the Denon AVP-A1HDCI (labeled “SSP” in the diagram above) and Oppo BDP-95 instead of the SSP-800 and Oppo BDP-105 that now inhabit his system. Not indicated in the diagram is the connection to the third sub, which reproduces the bass from the center and surround channels.

What’s the big deal about stereo bass? Aren’t low frequencies non-directional? Well, yes, but the frequency at which sound becomes directional is not a well-defined threshold—it becomes gradually more directional as the frequency increases.

More importantly, there are often phase differences in the bass in the left and right channels, which contribute to the sense of ambient space in the recording. This is most evident with live classical and jazz recordings and not so much with studio-created pop recordings in which each instrument is recorded mono and panned more or less to the left or right.

Most movie soundtracks are created in much the same way as studio recordings—that is, each source of sound is recorded in mono and panned around the surround soundfield. So having stereo subs isn’t important in that respect. However, having two subs is important because that lets you place them to minimize room modes.

But in this case, the best placement for the subs is not necessarily the best placement for the main speakers, which is why I normally recommend separate subs for movie sound. On the other hand, if you want to use those two subs for stereo bass, they need to be placed near the front left and right speakers, which sometimes defeats the purpose of two subs for movie sound.

Regarding auto EQ/room correction, Audyssey Sub EQ HT treats two subwoofers separately, pinging each one to determine the delay and level differences, then pinging them together as “one” after those differences are adjusted to create a room-correction filter. But in terms of program material, both subs normally receive the same signal.

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